JP2008010676A - Organic thin film transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic thin film transistor capable of achieving a field effect mobility as high as that in a conventional Au source electrode by using an inexpensive material without using an expensive material such as Au for the source electrode or the drain electrode. <P>SOLUTION: This organic thin film transistor comprises a gate electrode 24 of an electrically conductive substrate 22, a gate insulating film 26 formed on one surface side of the substrate, the source electrode 24 and the drain electrode 30 formed on the gate insulating film so that they are mutually spaced apart, and an organic semiconductor layer 32 connected to the source electrode and the drain electrode. The neighboring region of the source electrode including the surface thereof consists of nickel oxide 30A, and the interior of the source electrode consists of nickel metal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an organic thin film transistor in which a gate electrode, an organic semiconductor film, a source electrode, a drain electrode and the like are formed on an insulating substrate.

  Conventionally, many field effect transistors are manufactured using an inorganic material such as amorphous silicon or polysilicon formed on a silicon single crystal substrate (wafer) or a glass substrate. However, it is difficult to increase the wafer size in the case of using a silicon single crystal substrate because of restrictions on the manufacturing apparatus, and is not suitable for increasing the area. Further, in order to form a thin film made of amorphous silicon or polysilicon, a heat treatment step of 350 ° C. or higher is required, so a heat-resistant substrate is required. Usually, a heat-resistant glass substrate is not used as the substrate material. Don't be.

  In addition, in order to manufacture these insulating layers and semiconductor layers for inorganic semiconductors, expensive plasma chemical vapor deposition (CVD) apparatuses, laser annealing apparatuses, etc. must be used, which increases the manufacturing cost. ing. As described above, it is difficult to reduce the cost of a field effect transistor using an inorganic semiconductor due to manufacturing restrictions.

  On the other hand, a field effect type organic thin film transistor using an organic semiconductor layer has been recently proposed (Patent Document 1 and Non-Patent Document 1). The organic thin film transistor using the organic semiconductor layer can be formed at a low temperature and have a large area as compared with an organic thin film transistor using an inorganic semiconductor, and can be reduced in cost because it is easy to manufacture. Here, the structure of a conventional general organic thin film transistor will be described with reference to FIG. FIG. 4 is a schematic diagram showing a conventional field effect type organic thin film transistor of the bottom contact type. As shown in FIG. 4, in this organic thin film transistor 2, a gate film 6 is formed on an insulating substrate 4, and a gate insulating film 8 is formed on the gate film 6 so as to cover it. The

  A source electrode 10 and a drain electrode 12 are formed on the gate insulating film 8 so as to be separated from each other. An organic semiconductor layer 14 connected to these electrodes 10 and 12 is formed to constitute the organic thin film transistor 2. Here, a bottom contact transistor in which both electrodes 10 and 12 are located closer to the insulating substrate 4 than the organic semiconductor layer 14 is shown. However, the organic semiconductor layer 14 is closer to the insulating substrate 4 than the both electrodes 10 and 12. A top contact type transistor located in the region is also known. The organic semiconductor layer 14 often uses pentacene, which is a low-molecular material, and this pentacene is a p-type organic semiconductor and has a mobility that is substantially the same as that of amorphous silicon (Si).

JP 2004-63975 A “Flexible semitransparent penentene thin-film transducers with polymer dielectric layers and NiOx electrodes” (Appl. Phys. Lett. 87, 023504 (2005)).

  By the way, in the organic thin film transistor as described above, gold (Au) is often used as the source electrode 10 and the drain electrode 12. However, the use of Au, which is a noble metal, for the source electrode 10 and the drain electrode 12 is contrary to the cost reduction characteristic of the organic thin film transistor.

In this case, it is reported that a conductive material such as ITO (indium tin oxide film) or NiOx (X: positive rational number) is used as a material for the source electrode 10 and the drain electrode 12 other than Au. The field effect mobility as high as the Au electrode cannot be obtained yet. In addition, since these conductive materials have a considerably higher electric resistance than metals, it is difficult to use them as circuit wiring materials.
The present invention has been devised to pay attention to the above problems and to effectively solve them. The object of the present invention is to provide a field effect mobility equivalent to the case of using a conventional Au source electrode by using a low-cost material without using an expensive material such as Au as a source electrode or a drain electrode. An object of the present invention is to provide an organic thin film transistor that can be used.

According to the first aspect of the present invention, there is provided a gate electrode which is a conductive substrate, a gate insulating film formed on one surface side of the substrate, and a source electrode and a drain formed on the gate insulating film so as to be separated from each other. An organic thin film transistor comprising: an electrode; and an organic semiconductor layer connected to the source electrode and the drain electrode, wherein the source electrode includes a region including a surface thereof made of nickel oxide and an inside made of nickel metal. It is.
According to a second aspect of the present invention, an insulating substrate, a gate electrode formed on one surface of the substrate, a gate insulating film covering the gate electrode, and a gate insulating film are formed apart from each other. A source electrode and a drain electrode, and an organic semiconductor layer connected to the source electrode and the drain electrode. The source electrode is made of nickel oxide on the surface and in the vicinity of the surface, and the inside is made of nickel metal. It is an organic thin-film transistor characterized by this.

  According to the organic thin film transistor of the present invention, the source electrode and the drain electrode are made of Ni instead of Au, which is an expensive noble metal material, and only the surface thereof is made of a NiOx film. In addition, the electrical resistance value can be kept as low as Ni. Further, this structure can be used for wiring of an electric circuit, and the cost can be reduced more than that of a conventionally used Au electrode.

Hereinafter, an example of an organic thin film transistor according to the present invention will be described in detail with reference to the accompanying drawings.
First, a first embodiment of the present invention will be described.
FIG. 1 is a view showing a manufacturing process of a first embodiment of the organic thin film transistor according to the present invention, and FIG. 1E is a cross-sectional view showing the organic thin film transistor of the first embodiment which is a finished product.

  First, the organic thin film transistor of the first embodiment will be described with reference to FIG. As shown in FIG. 1E, the organic thin film transistor 20 has a thin substrate 22 on which a gate electrode 24 is formed. The gate electrode 24 is made of, for example, a low resistance impurity-doped silicon film. In this case, for example, an insulating substrate is used as the substrate 22. A gate insulating film 26 is formed on the substrate 22 so as to bury the gate electrode 24. The gate insulating film 26 is made of, for example, a silicon oxide film.

  A source electrode 28 and a drain electrode 30 are formed on the gate insulating film 26 so as to be spaced apart to have a predetermined channel length. Here, at least the source electrode 28 of the source electrode 28 and the drain electrode 30 is made of nickel (Ni) metal, and the surface thereof is a nickel oxide film (NiOx: x is a positive rational number) made of nickel oxide. ) 28A. Here, not only the source electrode 28 but also the drain electrode 30 is made of nickel metal, and its surface is made of a nickel oxide film 30A.

  An organic semiconductor layer 32 connected to the electrodes 28 and 30 is formed on the channel portion, which is a gap between the source electrode 28 and the drain electrode 30, whereby the organic thin film transistor of the first embodiment is formed. 20 has been completed.

Next, a method for manufacturing the organic thin film transistor 20 of the first embodiment will be described.
First, as shown in FIG. 1A, a silicon film doped with an impurity as a gate electrode 24 and having a low resistance was formed on a substrate 22 made of an insulating glass plate.
Next, a silicon oxide film was formed as a gate insulating film 26 on the substrate 22 so as to embed the gate electrode 24. The thickness of the gate insulating film 26 is, for example, about 400 nm.

  Next, in order to fabricate a source / drain electrode pattern having a channel length of 200 μm and a channel width of 3 mm, a negative resist (ZPN1150) is applied using a spin coating method, post-baked at 90 ° C. for 90 seconds, and then aligned. Then, a Cr mask for forming source / drain electrode patterns was set and exposed. And at 85 ° C. for 60 seconds, Post. Exposure Bake (PEB) was performed and developed using a developer to prepare a resist pattern for forming source / drain electrodes. Ni is deposited on the resist pattern by a vacuum deposition method so that the thickness is 60 nm, and the resist pattern is removed by a lift-off method, whereby a source electrode 28 and a drain electrode are formed as shown in FIG. 30 were formed respectively.

Thereafter, the entire substrate is subjected to oxygen plasma treatment at 400 W for 8 minutes with an ashing apparatus to oxidize the vicinity of the outermost surface including the outermost surfaces of the source electrode 28 and the drain electrode 30 made of Ni, and the result shown in FIG. Thus, NiOx films 28A and 30A were formed. By depositing pentacene as the organic semiconductor layer 28 on this substrate, an organic thin film transistor 20 as shown in FIG. 1E was completed. The pentacene evaporation rate was 0.05 nm / s and the thickness was 50 nm. At this time, the field effect mobility of the produced field effect type organic thin film transistor 20 was 0.36 cm ² / Vs. It was confirmed that the field effect mobility of the organic thin film transistor 20 was the same value as that produced using an Au electrode.

  In the case shown in FIG. 1, the case where the substrate 22 is formed of, for example, an insulating glass substrate has been described as an example, but instead of this, for example, a semiconductor substrate is formed as in a modification shown in FIG. A conductive semiconductor substrate 23 doped with impurities may be used. According to this, the entire conductive semiconductor substrate 23 can be used as the gate electrode 24, and it is not necessary to provide the gate electrode 24 at the position shown in FIG.

Next, a second embodiment of the present invention will be described.
FIG. 3 is a view showing a manufacturing process of the second embodiment of the organic thin film transistor according to the present invention, and FIG. 3F is a cross-sectional view showing the organic thin film transistor of the second embodiment which is a finished product. The same components as those shown in FIG. 1 will be described with the same reference numerals.

First, the organic thin film transistor of the second embodiment will be described with reference to FIG. As shown in FIG. 3F, the organic thin film transistor 40 has a thin substrate 22 and a base film 42 for enhancing adhesion to the gate electrode is formed on the upper surface. For example, a silicon oxide film can be used as the base film 42. A gate electrode 24 is formed on the base film 42. As the gate electrode 24, for example, tantalum (Ta) metal is used. A gate insulating film 26 is formed so as to cover and surround the gate electrode 24. As the gate insulating film 26, for example, a tantalum oxide film (Ta ₂ O ₅ ) can be used.

    A source electrode 28 and a drain electrode 30 are formed on both sides of the gate insulating film 26 so as to be spaced apart to have a predetermined channel length. Here, at least the source electrode 28 of the source electrode 28 and the drain electrode 30 is made of nickel (Ni) metal, and the surface thereof is made of a nickel oxide film (NiOx: x is a positive rational number) 28A. ing. Here, not only the source electrode 28 but also the drain electrode 30 is made of nickel metal, and its surface is made of a nickel oxide film 30A.

  An organic semiconductor layer 32 connected to both the electrodes 28 and 30 is formed on at least a channel portion that is a gap between the source electrode 28 and the drain electrode 30. The thin film transistor 40 is completed.

Next, a method for manufacturing the organic thin film transistor 40 of the second embodiment will be described.
First, as shown in FIG. 3A, a SiO ₂ film having a thickness of 150 nm is formed on a substrate 22 made of a glass substrate 20 by RF sputtering as a base film 42 for enhancing the adhesion of a gate film. Next, a Ta film having a thickness of 250 nm is formed on the base film 42 by RF sputtering at a sputtering pressure of 4 Pa. Then, a positive resist (OFPR800) is applied by spin coating, pre-baked at 110 ° C. for 90 seconds, contact exposed using a Cr mask having a desired gate shape with an aligner, and subsequently developed to develop a gate-shaped resist pattern. Form. Next, only the Ta film of the resist is left by RIE (Reactive Ion Etching), and a gate electrode 24 made of a Ta film is formed as shown in FIG.

Next, removing the resist on the gate electrode 24 made of Ta film, Ta ₂ O ₅ with a thickness of about 150nm from the surface is oxidized by anodic oxidation of the Ta film surface, as shown in FIG. 3 (C) A film is formed to form the gate insulating film 26. The following is performed in the same manner as in the first embodiment. As shown in FIGS. 3C to 3F, the source electrode 28 and the drain electrode made of Ni metal whose surfaces are covered with NiOx films 28A and 30A, respectively. 30 was formed, and the organic semiconductor layer 32 connected to both the electrodes 28 and 30 was formed, whereby the organic thin film transistor 40 was completed. At this time, in the field effect type organic thin film transistor 40 produced, a field effect mobility of 0.40 cm ² / Vs can be obtained, and this field effect mobility should be higher than in the case of the first embodiment. I was able to.

  In the first and second embodiments described above, the reason why the field effect mobility is increased is that the source electrode 28 and the drain electrode 30 made of Ni are subjected to oxygen plasma treatment so that the surface of the Ni film and the vicinity thereof are NiOx films 28A and 30A. By doing so, it is presumed that the field effect mobility increased because the work function increased and the hole injection efficiency into the organic semiconductor layer 32 made of pentacene increased. As a result of measuring the work functions of the source electrode 28 and the drain electrode 30 with a photoelectron spectrometer (RIKEN KEIKI: AC-1), the work function of the Ni film immediately after deposition was 4.62 eV. As a result, it was confirmed that the voltage was increased to 5.9 eV.

  The thickness of the NiOx film formed by the oxygen plasma treatment of the first and second embodiments is about 5 nm. The film thickness can be reduced to about 1 nm, but if it is reduced below that, the hole injection efficiency decreases, so the thickness of the NiOx films 28A and 30A is preferably 1 nm or more. Further, although the source electrode 28 is made of the NiOx film 28A only on the surface, most of the source electrode 28 is made of Ni metal and maintains a high conductivity state. Therefore, it is possible to share this Ni wiring not only for the source electrode 28 and the drain electrode 30 but also for the wiring portion of the circuit. In order to maintain conductivity, it is necessary to keep the NiOx film up to half of the total thickness of the Ni film formed initially. Further, considering the efficiency of the method of oxidizing the Ni film surface, the thicknesses of the NiOx films 28A and 30A must be 50 nm or less, respectively.

  The Ni film may be formed by a sputtering method, a plating method or the like in addition to the methods described in the first and second embodiments. The film thickness may be set in the range of about 5 nm to 500 nm so that the electric resistance is not so high. Further, as a method for oxidizing the Ni film surface to form a NiOx film, a wet method such as an atmospheric pressure oxygen plasma treatment, a dry method such as a UV ozone method, or an anodic oxidation method may be used. However, the surface roughness Ra of the obtained film needs to be 10 nm or less. When the surface roughness is too large, the contact resistance with pentacene increases, and high field effect mobility cannot be obtained.

In the first and second embodiments, the drain electrode 30 has the same NiOx / Ni configuration as the source electrode 28. However, the material of the drain electrode 30 may be a conductive material and is limited to the current material. It is not a thing.
Moreover, in the said 1st and 2nd Example, the board | substrate 22 is not limited to what was demonstrated in the Example, Both an inorganic substrate and an organic substrate can be used. Specifically, any substrate of an inorganic material having excellent surface smoothness such as a glass substrate or Si substrate can be used. A plastic substrate made of an organic material such as polyethylene terephthalate (PET), polystyrene (PS), or polyethersulfone (PES) can also be used.

  The gate electrode 24 is a low-resistance Si film or Ta film, but is not limited to the embodiment as long as it has conductivity. Further, the gate insulating film 26 is desired to have a high insulating property and a high relative dielectric constant. Therefore, it is not limited to an inorganic material, and an organic insulating film is also possible. Specifically, a thin film of silicon oxide, silicon nitride, aluminum oxide, titanium oxide, zircon oxide, or the like can be used by an evaporation method, a sputtering method, a CVD method, or the like. In addition, thin films such as polyethylene, polyvinyl carbazole, polyimide, polyparaxylene, and the like can be used by spin coating, LB monomolecular accumulation, or the like. The film thickness of these gate insulating films is often about 10 nm to 1000 nm.

Further, the organic semiconductor layer 32 is formed by a condensed aromatic hydrocarbon such as pentacene, tetracene, or perylene, a derivative of these condensed aromatic hydrocarbons, and a polymer material such as polyacetylene, by vapor deposition, spin coating, ink jet, or the like. Conjugated hydrocarbon polymers such as polyacene, conjugated heterocyclic polymers such as polyaniline, polypyrrole, and polythiophene can be used.
According to the present invention, it is possible to obtain a field effect organic thin film transistor having the same performance as an Au electrode by using an inexpensive Ni electrode and oxidizing only the outermost surface without using an expensive Au electrode. Since only the Ni surface is oxidized, it can also be used for the electrode wiring itself of the circuit.

It is a figure which shows the manufacturing process of 1st Example of the organic thin-film transistor which concerns on this invention. It is sectional drawing which shows the modification of the organic thin-film transistor which concerns on this invention. It is a figure which shows the manufacturing process of 2nd Example of the organic thin-film transistor which concerns on this invention. It is a schematic block diagram which shows the conventional field effect type organic thin-film transistor of a bottom contact type.

Explanation of symbols

DESCRIPTION OF SYMBOLS 20 ... Organic thin-film transistor, 22 ... Substrate (insulating), 23 ... Substrate (conductive), 24 ... Gate electrode, 26 ... Gate insulating film, 28 ... Source electrode, 28A ... NiOx film, 30 ... Drain electrode, 30A ... NiOx Film 32 organic semiconductor layer 40 organic thin film transistor

Claims (2)

A gate electrode which is a conductive substrate;
A gate insulating film formed on one side of the substrate;
A source electrode and a drain electrode formed on the gate insulating film so as to be spaced apart from each other;
An organic semiconductor layer connected to the source electrode and the drain electrode,
The organic thin film transistor characterized in that the source electrode has a region including the surface thereof made of nickel oxide and the inside made of nickel metal. An insulating substrate;
A gate electrode formed on one side of the substrate;
A gate insulating film covering the gate electrode;
A source electrode and a drain electrode formed on the gate insulating film so as to be spaced apart from each other;
An organic semiconductor layer connected to the source electrode and the drain electrode,
The organic thin film transistor, wherein the source electrode has a surface and a portion near the surface made of nickel oxide, and an inside made of nickel metal.

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